This invention is directed to novel synthetic water-soluble polymers and their use as surfactant-mobility control agents in oil recovery processes.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
One difficulty often encountered in waterflooding operations is the relatively poor sweep efficiency of the aqueous displacing medium; that is, the injected displacing medium tends to channel through certain portions of the reservoir as it travels from the injection system to the production system and to bypass other portions. Such poor sweep efficiency or macroscopic displacement efficiency may be due to a number of factors such as differences in the mobilities of the injected displacing liquids and the displaced reservoir oil and permeability variations within the reservoir which encourage preferential flow through some portions of the reservoir at the expense of other portions.
Various techniques have been proposed in order to improve the sweep efficiency of the injected displacing medium and thus avoid premature breakthrough at one or more of the wells comprising the production system. The most widely used procedure involves the addition of thickening agents to the injected displacing medium in order to increase the viscosity thereof and thus decrease its mobility to a value equal to or less than the mobility of the displaced reservoir oil, resulting in a "mobility ratio" of oil to water which is less than or equal to one. Many polymeric thickening agents including both anionic and cationic polyelectrolytes have been proposed for use in such mobility control operations. Thus, U.S. Pat. No. 3,085,063 discloses waterflooding in which the water is thickened by the addition of polyvinyl aromatic sulfonates such as sulfonated polystyrene and copolymers of such vinyl aromatic sulfonates. Similarly, U.S. Pat. No. 3,984,333 discloses waterflooding involving the injection of an aqueous solution thickened by block copolymers in which the water-soluble blocks are sulfonated polyvinylarenes and the relatively water-insoluble blocks are polymerized alpha olefins and/or hydrogenated dienes such as polyisoprene and polybutadiene. Synthetic anionic polymers such as the widely used partially hydrolyzed polyacrylamides, suffer a number of disadvantages in actual operations. Where the injected water or the reservoir water contains as little as 0.5% of dissolved inorganic salts, this viscosity yield is decreased more than 80 %. Also U.S. Pat. No. 3,969,592 discloses water-soluble polymers manufactured by treating an aqueous suspension of protein with selected enzymes.
Also U.S. Pat. No. 4,110,232 discloses a waterflooding process for producing oil wherein an aqueous solution of a copolymer formed of hydrophobic olefinic segments and cationic aromatic segments is injected into oil-containing subterranean formation, and U.S. Pat. No. 4,222,881 discloses a waterflood oil recovery process involving the use of an amphoteric polyelectrolyte as a thickening agent for mobility control comprising a copolymer of a quaternary vinyl pyridinium sulfonate-styrene block copolymers.
Also U.S. Pat. No. 4,284,517 and U.S. Pat. No. 4,317,893 describe an anionic polymeric surfactant and viscosifier formed by reacting a polymer including succinic anhydride moieties with a primary amine to provide a polymeric reaction product in which at least 20 mol percent of the anhydride moieties have been converted, by reaction with said amine, to succinimide or succinamide groups.
Most water-soluble polymers lose their thickening power in the presence of salts or other electrolytes. The loss of thickening power is due to the change of conformation of the polymer chains. In distilled water, due to charge repulsion of neighboring groups, the polymer chains exist in a highly extended conformation which results in a large hydrodynamic volume and consequently a high solution viscosity. In the presence of excess small electrolytes, due to charge screening the polymer chains exist in a coiled conformation which results in a small hydrodynamic volume and consequently a low solution viscosity. In oil recovery from most reservoirs the polymers must function under a brine environment. To date certain polysaccharides, such as xanthan gum, have been found to maintain a reasonable viscosity in high brine solutions. This solution property of the polysaccharides appears to be attributable to the transformation to the stiff helical conformation in the presence of electrolytes. The usual more flexible water-soluble polymers coil and become more compact in the presence of electrolytes, consequently lose water thickening power. However, the above-described polysaccharides easily hydrolytically degrade through the unstable hemiacetal linkages in the polymer main and side chains.